A major hazard in water-cooled nuclear reactors is the accumulation of radioactive substances in the structural portions of the reactor. The buildup of radioactive nuclides occurs on the inner surfaces of components which are in contact with the reactor water. This includes both the primary recirculation circuit and the reactor water cleanup system.
During reactor shutdown, workers are exposed to radiation emanating from stainless steel internal walls and inner surfaces of piping. Radioactive materials retained in oxide films which have accumulated on wall and piping surfaces are a major source of radiation exposure. The radioactivity has been found to be predominantly due to the Co-60 isotope. As a result, a substantial effort has been made to identify the key parameters which affect Co-60 buildup and to determine and implement methods for limiting that buildup.
Cobalt, derived from a number of different alloys commonly employed in components of the reactor's mechanisms or structures, is subject to induced radioactivity, especially the Co-60 isotope. This radioactive Co-60 isotope, or ions or compounds thereof, can be carried in the circulating cooling water flowing through the cooling water circuit, whereby the radioactive substances are spread and deposited throughout the cooling water circuit or primary loop system of the reactor plant. Such radioactive substances are prone to be taken up and incorporated into the normally occurring oxide films which form and progressively accumulate on the inner surfaces of the cooling water circuit.
A number of proposals or potential solutions to this problem of dispersion of radioactive substances throughout the cooling water circuit or system and incorporation of radioactive substances into the inherently produced oxide films forming on the surfaces of cooling water circuit or system have been considered or made. One approach to controlling the potentially hazardous cobalt source of such radiation has been the application of zinc as disclosed in U.S. Pat. No. 4,756,874 to Ruiz et al. and U.S. Pat. No. 4,759,900 to Peterson et al.